Terrestrialness is a noun that describes things relating to the earth or land. Antonyms for terrestrialness could include things that are aquatic or sea-related, such as marine, oceanic, or aquatic. They could also include things that are airborne or atmospheric, such as aerial or atmospheric. Other potential antonyms might include things that are extraterrestrial or celestial, such as lunar, cosmic, or extraterrestrial. Finally, antonyms for terrestrialness might include things that are completely abstract or intangible, such as ethereal, spiritual, or imaginary. Overall, there are many possible antonyms for terrestrialness, depending on the context in which the word is being used.